1. Field of the Invention
The present invention relates to a digital camera, more particularly to an image display device in the digital camera.
2. Background of the Art
A ⅓ through ½ type charge-coupled device (CCD) has been widely used in a digital camera. With a decrease of a pixel size, such a ⅓ through ½ type CCD tends to have a large number of pixels, such as, 3 through 5 million pixels. On the other hand, a digital camera includes a display device, such as, a liquid crystal display (LCD) having about 0.1 million pixels. The size of an LCD has been required to be reduced due to demands for the decrease of the size of a digital camera. For this reason, an LCD has a relatively small number of pixels compared to a CCD.
A CCD of interlace type has been mainly used due to the merit in reducing the size of a CCD. In the CCD of interlace type, due to the decrease of capacity of a potential well in a vertical transfer path, the CCD of interlace type tends to read out overall pixels by dividing a frame into three or four fields rather than by dividing a frame into two fields.
In a CCD of primary color filter type that reads out pixels in an odd number of fields such as three fields, three primary color (red, green, blue) pixels can be read out from one field. Therefore, necessary information for preparing a color image is obtained only from the one field. For example, when using a 5 million pixel CCD having an aspect ratio of 4 to 3, the number of horizontal lines used for recording is generally 1944. When reading out overall pixels by dividing a frame into three fields, the number of horizontal lines used for recording per one field is 648 (i.e., 1944×⅓).
Generally, a CCD is driven by at least two drive modes, that is, a draft mode and a frame mode. In the draft mode, lines for reading out pixels in the vertical direction are limited for framing at the time of monitoring. The number of such limited lines is about 250 that is close to the number of lines of single field for TV picture signals. A refresh speed of a frame (time for reading out pixels in one frame) is set to from about 1/60 to about 1/15 seconds for smooth framing. In the frame mode, overall pixels are read out. For example, when using the above-described 5 million pixel CCD, time for reading out pixels in one field is from about 1/20 to ⅕ seconds unless a horizontal drive frequency is changed. This is because the number of horizontal lines for reading out pixels per one field in the frame mode is 648 which is about three times of the number of lines for reading out pixels in the draft mode (i.e., about 250).
As described above, many CCDs have been used, which obtain images of different resolutions with different number of horizontal lines at a relatively high speed while changing drive modes for CCDs. In the case of an imaging device that can perform a random access, such as, a complementary metal oxide semiconductor (CMOS) sensor, such an imaging device can change the resolution of an image output from the imaging device by cutting any desired part of pixels out of overall pixels in an angle of view. However, a digital camera rarely uses a CMOS sensor as an imaging device due to a problem such as a fixed pattern noise.
In this specification, a term of “resolution” indicates the number of pixels in a vertical or horizontal direction. Especially, the number of pixels in a vertical direction may be referred to as the number of lines.
As described above, an imaging device can perform monitoring with different resolutions at a relatively high speed while changing drive modes, such as, a draft mode and a frame mode. In view of this situation, when an image is output to an LCD having about 200 horizontal lines, the image has no problem with its resolution even if pixels are read out by the draft mode. The monitoring by the frame mode is not usually considered because of high consumption of electric power.
However, when an operator of a digital camera confirms a focus condition of an image, the image is required to be monitored with high resolution. There is a background digital camera having a manual focus function in which an operator adjusts a focus position while monitoring an image output to an LCD. In such a digital camera, an image is enlarged and displayed on a part of a screen of an LCD for confirming a focus condition of the image. Such a digital camera is described, for example, in published Japanese patent application Nos 11-196301 and 11-298791. There is another background digital camera having an auto focus function in which a part of an image or a focus area is temporarily enlarged and displayed on a part or the whole of a screen of an LCD for confirming a focus condition when an auto focus operation is completed. Such a digital camera is described, for example, in published Japanese patent application No. 2001-211351.
In these background digital cameras, an image is enlarged and displayed by an image processing large-scale integration (LSI) while driving a CCD by a draft mode. In this condition, the resolution of picture signals obtained from an imaging device (CCD) is low with respect to that of a display device (LCD). Therefore, even though an image is enlarged, an operator cannot confirm a focus condition clearly.
There is a background technique in which overall pixels are always read out from an imaging device having pixels greater than those of a display device. Then, readout pixels are thinned out by an image processing LSI. For example, Japanese patent application No. 11-298791 describes this technique. When using a 1.5 million pixel CCD of progressive type, overall pixels can be read out in about 1/7.5 seconds. A recently-used 3 million pixel CCD does not have such a progress type yet. Assuming that a 3 million pixel CCD of progressive type exists, it takes double time to read out overall pixels than 1.5 million pixel CCD of progressive type even if the CCD is horizontally driven at a maximum frequency allowed by an imaging device. In addition, the consumption of electric power of periphery IC for driving the CCD at a high speed becomes constantly high. On the other hand, if a horizontal drive frequency is decreased, the consumption of electric power is decreased. However, a refresh rate of a display during monitoring decreases, thereby deteriorating operability of a digital camera.